[graemep]

No, they are not used interchangeably, even in your examples.

Johnson is giving London as an example of a place in the UK. It is a very apposite example because it is where any power the police are given would be used most heavily (as stop and search is). He is using the the phrase "freeborn Englishman" to evoke associations with the use of the phrase that is far older than the UK. The phrase in its literal sense also excludes women and serfs and I do not think Johnson is aiming for that.

Brooke was writing a poem. He was evoking an effect and a particular sense of place and identify. "England" evokes different associations and and a different emotional response to "the united Kingdom". It is more local, and cosy, and has visual associations (the "English countryside" vs "the Scottish countryside" for example).